Classification of Stars

The brightness of a star depends on its temperature. Combined with the star’s chemical composition this will determine its so-called “stellar spectra”.

Antares, the sixteenth brightest star in our night sky (centre left of this image of the Antares region).

The first attempt to classify stars based on their stellar spectra was made by an Italian astronomer (and scientific adviser to the Pope), Father Pietro Angelo Secchi. He divided spectra into three classes:

  • Class I     White stars having strong hydrogen lines, such as Vega (Alpha Lyrae).
  • Class II    Yellow stars having strong metallic lines, such as Arcturus (Boötes).
  • Class III   Red stars having broad, fluted bands of molecules such as titanium oxide, such as Antares (Alpha Scorpii).

By 1900 this classification had been extended by the Harvard College Observatory to provide for six spectral types, namely B, A, F, G, K and M. Later the system was further expanded to add W and O types before type B, and to subdivide G and K, to create the following schema:

Stella Spectra
Side branchesC N OC R NS
Note that 99% of all stars fall into the B to M type range

The reason the types are not in alphabetical order is simply historical. It started out that way (A-M), but with continual discoveries the list had to be extended as you see above, being the temperature sequence of spectral types of stars with “W” and “O” representing the hottest stars and “M” the coldest stars.

Type D stars are “dead stars”,  white dwarfs that are no longer producing energy. They have an estimated age range of 100 000 years to 10 billion years. They are small, often less than the size of Earth, but extremely dense, with temperatures ranging from around 7 700°C to 40 000°C. Neutron stars are also Type D.

You might need a mnemonic to remember the correct order—just remember “W” now comes first. It stands for Wolf-Rayet, by the way, and these massive stars are very rare. So try: Oh Be A Fine Girl and Kiss Me Dead!

So why is blue hot? After all we tend to think of red as being hot—think of sunburn or think of passion; and blue being cold (your lips or fingers turn blue when cold). But in space life is very different where the colours are based on the wavelength of light. The energy in a photon depends on its wavelength: the shorter the wavelength the more energetic the photon (so it looks blue); the longer the wavelength the less energetic the photon (so it looks red). This is why blue stars are “hot” and red stars are “cool”.

Properties of each star type
Spectral typeTemperature °CProminent spectral linesColourExample
W30 000 - 210 000Ionised helium and highly ionised nitrogen and carbonBlue(no-name brand)
O25 000 - 40 000Ionised helium; oxygen, nitrogen, carbon and siliconBlueRegor
B11 000 - 25 000Helium and some hydrogen, carbon, oxygen, nitrogen, iron and magnesiumBluish-whiteRigel
A7 500 - 11 000Strong hydrogen, some ionised metals such as calciumWhiteSirius
F6 000 - 7 500Hydrogen, ionised calcium and ironYellowish-whiteProcyon
G5 000 - 6 000Calcium, iron, titanium, magnesium, hydrogen and some molecular bandsYellowSun
K3 500 - 5 000Calcium, hydrogen, some sodium. and molecular bands such as titanium oxideOrangeAlderbaran
M2 500 - 3 500Strong titanium oxide, very strong sodium, calcium and molecular bandsRedAntares
See text for further details; some of the elements are ionised particles

By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Thursday, 18 March 2021

Back to Top